1. Field of the Invention
The present invention generally relates to a back-light module of image scanning devices for transmissive original documents, and in particular to a method for calibration of illumination in order to obtain a substantially uniform illumination over an original document.
2. Description of the Prior Art
Document scanners are generally classified in two types for respectively handling a reflective original document which comprises an opaque substrate and a transmissive original document which comprises a transparent substrate. A transmissive original document scanner comprises a back-light module for generating light projecting the image formed on an original onto an image sensor system of the document scanner.
A conventional back-light module comprises a movable line-type light source which is moved in a given direction from one end of the original document to an opposite end. A driving system is required to move the light source which complicates the overall structure of the back-light module.
Another conventional back-light module comprises a surface-type light source which requires no movement of any parts of the back-light module. FIG. 1 of the attached drawings shows an image scanning device having a back-light module comprising a surface-type light source and FIG. 2 shows an exploded view of the back-light module.
As shown in FIG. 1, a conventional scanner for transmissive original documents, designated with reference numeral 1, comprises a housing (not labeled) having a transparent document supporting plate 10 for supporting an original document (not shown) containing an original and a back-light module 3 in the form of a flip cover for selectively covering the document supporting plate 10. An optical scanning module 11 comprising a sensor system is movably supported inside the housing by guide rails 12, 13 for moving in a longitudinal direction (Y direction) under the control of a control unit 14. The sensor system comprises a line of sensing elements, such as an array of CCD (Charge Coupling Device), arranged in a lateral direction (X direction) onto which a “scan line” of the original is projected by light generated by the back-light module 3. The sensing elements convert the optical signal caused by the scan line into electrical representation of the scan line. By moving the optical scanning module in the longitudinal direction line by line or step by step and scanning the original one scan line at a time, the original or a portion of the original document may be scanned.
As shown in FIG. 2, the back-light module 3 comprises a casing 31 having an open bottom closed by a bottom plate 32. An opening 32a is defined in a central area of the bottom plate 32 receiving and retaining a transparent plate 37. Two tubular lamps 41, 42, such as cold cathode fluorescent lamps, are arranged inside the casing 31 and spaced from each other with a light guide plate 34 disposed therebetween. Two diffusion boards 35, 36 are arranged between the light guide plate 34 and the transparent plate 37. Light from the tubular lamps 41, 42 are guided by the light guide plate 34 for spreading over and projecting from the light guide plate 34 onto the diffusion boards 35, 36. The light is further distributed by the diffusion boards 35, 36 to uniformly project toward and through the transparent plate 37. A reflective sheet 33 is arranged between the light guide plate 34 and a top of the casing 31 for directing light back to the transparent plate 37.
Since uniform distribution of light is required in obtaining good result of scanning transparent original documents, the diffusion boards 35, 36 are important parts for the conventional scanner. Although an illumination calibration zone 2 extending in the direction of the CCD array, namely the X direction (or the lateral direction as defined above), for calibration of illumination of the back-light source, there is no way in the conventional design to calibrate illumination in the Y direction (or the longitudinal direction as defined above). Uniformity of illumination in the Y direction is in generally achieved by the diffusion boards 35, 36. However, using diffusion boards to uniformly distribute light complicates the overall structure of the back-light module and increases costs.
Thus, it is desired to provide a back-light module of an image scanning device for overcoming the above discussed problems.